docs: def _process_omim2gene(self, limit=None):
          """
          This method maps the OMIM IDs and KEGG gene ID.
          Currently split based on the link_type field.
          Equivalent link types are mapped as gene XRefs.
          Reverse link types are mapped as disease to gene associations.
          Original link types are currently skipped.
  
          Triples created:
          <kegg_gene_id> is a Gene
          <omim_gene_id> is a Gene
          <kegg_gene_id>> hasXref <omim_gene_id>
  
          <assoc_id> has subject <omim_disease_id>
          <assoc_id> has object <kegg_gene_id>
          :param limit:
  
          :return:
          """
  
          LOG.info("Processing OMIM to KEGG gene")
          if self.test_mode:
              graph = self.testgraph
          else:
              graph = self.graph
          model = Model(graph)
          line_counter = 0
          geno = Genotype(graph)
          raw = '/'.join((self.rawdir, self.files['omim2gene']['file']))
          with open(raw, 'r', encoding="iso-8859-1") as csvfile:
              filereader = csv.reader(csvfile, delimiter='\t', quotechar='\"')
              for row in filereader:
                  line_counter += 1
                  (kegg_gene_id, omim_id, link_type) = row
  
                  if self.test_mode and kegg_gene_id not in self.test_ids['genes']:
                      continue
  
                  kegg_gene_id = 'KEGG-' + kegg_gene_id.strip()
                  omim_id = re.sub(r'omim', 'OMIM', omim_id)
                  if link_type == 'equivalent':
                      # these are genes!
                      # so add them as a class then make equivalence
                      model.addClassToGraph(omim_id, None)
                      geno.addGene(kegg_gene_id, None)
                      if not DipperUtil.is_omim_disease(omim_id):
                          model.addEquivalentClass(kegg_gene_id, omim_id)
                  elif link_type == 'reverse':
                      # make an association between an OMIM ID & the KEGG gene ID
                      # we do this with omim ids because
                      # they are more atomic than KEGG ids
  
                      alt_locus_id = self._make_variant_locus_id(kegg_gene_id, omim_id)
                      alt_label = self.label_hash[alt_locus_id]
                      model.addIndividualToGraph(
                          alt_locus_id, alt_label, self.globaltt['variant_locus'])
                      geno.addAffectedLocus(alt_locus_id, kegg_gene_id)
                      model.addBlankNodeAnnotation(alt_locus_id)
  
                      # Add the disease to gene relationship.
                      rel = self.globaltt['is marker for']
                      assoc = G2PAssoc(graph, self.name, alt_locus_id, omim_id, rel)
                      assoc.add_association_to_graph()
  
                  elif link_type == 'original':
                      # these are sometimes a gene, and sometimes a disease
                      LOG.info(
                          'Unable to handle original link for %s-%s',
                          kegg_gene_id, omim_id)
                  else:
                      # don't know what these are
                      LOG.warning(
                          'Unhandled link type for %s-%s: %s',
                          kegg_gene_id, omim_id, link_type)
  
                  if (not self.test_mode) and (
                          limit is not None and line_counter > limit):
                      break
  
          LOG.info("Done with OMIM to KEGG gene")
  
          return